To investigate the effect of a 6-week tai chi (TC) training program on physical performance in adolescents with Down syndrome and its influence on the psychological well-being of their parents, in a randomized controlled design, 25 male adolescents with Down syndrome (age 14.4 ± 1.30 years) were randomly assigned to a control group (n = 10) or a training group (n = 15). Before and after the training period, lower limb explosive strength, upper limb strength, flexibility, and balance were assessed in all participants, as well as their parents’ psychological well-being. Using 2 × 2 repeated-measures analysis of variance, significant Group × Time interactions (p < .05; $.33<{\mathrm{\eta }}_{\mathrm{p}}^2<.87$) were found for physical measures and parents’ depression, anxiety, and stress symptoms. A 6-week TC program significantly improved lower limb explosive strength (p < .001; d = 1.21), upper limb strength (p < .001; d = 1.49), flexibility (p < .001; d = 1.11), and static balance (p < .001; d = 1.99) and reduced depression (p < .001; d = 1.89), anxiety (p < .001; d = 1.74), and stress scores (p < .001; d = 1.88) in parents in the training group compared with those in the control group. TC programs improve physical measures in adolescents with Down syndrome and psychological well-being of their parents. Establishing TC programs in sport associations could positively impact this population’s physical performance.